Electrostatic loud speaker



March 22, 1932. A, A 4THCMAS 1,850,855

INVENTOR l im parts are housed in a suitable casing open for tached. tothe side oj frame Flhas a screwgQB 'm g 25 a simpliied'embodiment ofinylinvention, I such as aluminum audits alloys, brass,t1n,ufa i Y 135'*Figis -avertical on lineQQfO-f Sllfef A rSingle ksheet-.oli dielectricmaterial Se 1T or other 1snr-lil'falcle fastening means secure Wray. Inseine instances Dlclfockgjli may be W the fraine or casing rigidig;7 tothe supportcastor `,rfi-lclcled finte-gral with the body ci' the 33Patented Mla- 22, 19.32` j I; y r- Amman, momsoemwycms n. Yy l l lnnnc'rosia'rm Lonnsrnsxiin Application led February 16, y195%9. Serial340,;7OY'

This invention 'is for a ,newlcudspcaker ci as `a single piece, ifdesired- The front 16y thefccndenser type characteriaedfvb; novel has anepeningl large enough. for. thev'frfee feat-mee et construction; whichlpre nce an passage of sound. VA. sheet of genie er Screen `instrument of`high,eiciency ,and improved 19- may Gevel the Opening 1.8. teyprotectthe.

8 operation. .A :preferred form of *my invcn-V parte m the casing, aswell esto improve the up tioncompnscsva pairrcf vibratcry sheets heldappearance of. the instrument. yThe `back underV yten/eden incloseelcctrostatic relation t0 Wall 15 is secured to the body of theltraine c a: fixed platecn f ippcstel sides thcreet. An ,OrcaSingF in.any pretcal wey, as byscews, elastic dielectric made as ,thin as.possible bolts and the like. `In some instances, the;

separates the two vibratory sheets'irom the 'Y Il1II1lDP/1i15v ymay loevconstructed as a sound, it*v fixed plate. Theftensioned; sheets aresuping boavditoemplify thevoluine of repre-A port'ed at two ends;onlg,zand thcy'constitute Vduction,as Will-presently be understood,

e. pair;v of freefedge loudspeaking da- A ,rigid metalplate QOis linedin Casing p hragms adapte 1 :to vibrate with snbstanby `means efscrcws2l, Qr Otherwise. Itis 1I tmlly equglamplitudepver their entireacousconvenienty to insect the screws Vthrouggjcf the 65- tiic areas.Another `*eaimmef of my inventionY SidS 13 Oithe frame intothe edges ci'plete comprises means Ifor :readily adjusting the 20, whicliccnstitutesa rfixed condensei vplate Y normal -tenscrref the, diaphragme to ccntrcladapted to be connected to konefsicle ,of a suit.- the qualityofrepnodnctionl krIlie condenser able koutput circuit. A binding postQQlfatf the passage .of sound tfrom'both diaphragrns. engaging the plate29where'by the latten .is Ihelcack of the casing 4may be closed by .areadily yconnected to, a circuit lead. The con.-y sounding'board whichis set infvibration by denser plate 2O canbemade of 'any suitableelectric conductivity,

the reark diaphragmfto ampliiy thetones, In metal having 'goed mayuseonlyuonediaphmgm. iron, and 'various other ymetals and alloys lThevarious novel vle'atnres and practical with which the electrician isfamiliar. The advantages ofrmyinvention will be under? opposite faces ofplate 2O are covered with stood from a description ci theaccompanyelastic dielectric material 24, which7 may be a'thin layer .ofrubber cement, a..-stretclied v Fi shows one'form'o nigyfnew electrn-Sheet of 'indiarubber, locsely'woven abric static 'londe'pealxerY',rthis" iviewy being partly like netting, cheese- -lotli and the likemrany broken lasa/ay and sectioned on yline lfl of other materialacting asY `n electric insulator Fig. '2 *for clearness; and sniciently elastictoyield nnderpncsf' Fiel; and Y may .be wound tant over the metal plate 20Fig; represents a 4fragmentary ses-:tionen to cover beth acesancl the.oppcstc ends 125,

line oFig, 1.; n 1 Y The outer end'o t e slieetgi-s then cemented n A"suitable base t1@ 'sup 'oi-'ts an` upright 0r ctherwise lined in fplacevtoliold the sheet y frame indicated as a whole b'yF, whichV in inpermanentlystretclied condition.- v 99 this instance is in the formetai.shallowacas-` An installatine` block 26 lis mounted in ,the

ing cemprisingja top 12, sides13,`a bottom lower pcrtion o 'casing F,being seemed. by

14,' a backwwall i5, and a front i6. Screws screws v(netaslneirrfl) lorin am] other practical @Fis preicnaialy :of inT casing Tlie lolcckgQ is.provided with two 1 sulating materialfsucli as wood, hardrubber,parallel longitudinal slots 'and 428 for ite.- baltelite,'fand othersoffia similar natur-@but ceiving the lower ends ezt a pairof lcxible itmay -alsofbe macleo'f zlnetaL Thetopl, metall slieets .or Vtl-fn. plates29er@ .BO arand held in pressure contact with the dielectric 24. Thesesheets are mounted for vibratory movement and function as loudspeakingdiaphragms. Cross-pins 31 passing through the block 26 hold the lowerends of the metal sheets 29 and 30 anchored in place. he pins 31 mayproject through the casing to receive a binding post 32 for connectingthe sheets 29 and 30 in cir-` cuit. If these to be connected to oppositesides they are insulated from each other and a binding post is providedfor each. In the upper portion of casing F are mounted two cr0ss-bars33y and 34, which are supported on fixed pins 35 and a rotary shaft 36for lateral movement. The shaft 36 has oppositely screwthreaded sections37 and 38 passing through correspondingly screwthreaded bushings 39 and40 in cross-bars 33 and 34. The ends of screw shaft 36 are journalled inthe front and rear walls of casing F. A wheel 41 fixed on the center ofshaft 36 projects out of the casing through a slot 42 for manualoperation. The cross-bars 33 and 34 have slots 43 and-44 in which theupper ends of the'vibratory metal sheets 29 and 30 are inserted andsecured by cross-pins 45 or otherwise. The front sheet 29 is kept vintensioned condition by means of a pair of transverse rods 46 mounted inthe side walls of casing F and similar rods 47 engage the rear sheet 30to hold it constantly under tension.

It is clear from Fig. 2 that the tensioning rods 46 and 47 maintain theelastic metal sheets 29 and 30 in firm pressure contact with thedielectric sheet 24, which is therefore always under slight compression.The practical advantage of this arrangement is that the sheets 29 and 30are held as closely as possible to the fixed condenser plate 20 withoutactually touching it. The sheets 29 and 30 constitute free-edgedloudspeaking diaphragms which vibrate when subjected to potentialvariations with diaphragm sheets are intended circuit for the condenserkplates, because such circuits are well understood by those skilled inthe art and they form no part of this invention. It is enough to saythat the fixed condenser plate 20 is connected to one side of an outputcircuit of proper Avoltage while the two vibratory sheets 29 and 30 yareconnected to the other side of the circuit. If a constant polarizingpotential is used in addition to the signal volt'- side of thepolarizing circuit. This will be clear to those famihar with radiocircuits.

of a circuit,

phragms are always in pressure contact with the dielectric increases thesensitiveness of the instrument. The vibratory diaphragm sheets 29 and30 may consist of any suitable metal or alloy, such as aluminum,duralumin, brass, steel, and others along the same line. If desired, theouter surface of each diaphragm may be covered with a thin sheet ofpaper, cloth or leather to soften the sound. The dielectric 24 may beattached to the inner faces of the diahpragm sheets instead of ingmounted on the fixed plate 20, but I prefer the latter arrangement asbeing simler.

p The quality and volume of reproduction may be controlled to a certainextent by varying the normal `tension of the elastic diaphragm sheets 29and 30. This is done by simply turning the hand wheel 41 in the properdirection. Looking at Fig. 2, it is evident that when the wheel 41 isturned to separate'the cross-bars 33 and 34, the tension on thediaphragms is increased, while a reverse rotation of the wheelcorrespondingly lessens the tension. During this transverse movement ofthe cross-barsl 33 and 34, the fixed pins v35 and the rotary screw-shaft36 act as guides. Instead of adjusting the tension of both vdiaphragmssimultaneously, a separate Wheel 41 and screw-shaft 36 may be providedfor each diaphragm. In a simpler form of my .invention 'thetension-adjusting arrangement may be dispensedv with and the cross-bars33 and 34 mounted in fixed position.

As previously mentioned, the back wall 15 of casing F may be in the formof an elastic sounding board for amplifyn'ng the `reproduction.Thefvibrations of the rear diaphragm 30 ywill create sound wavesimpinging against the sounding board and setting it in vibration. Thespace back of diaphragm 30 may be open'to' the outer air through anopening in back wall 15 or through sound passages at the sides of plate20 leading to the front opening 18. In this way the acoustic outputofboth diaphragms comes through the open front of the casing. It ispossible to use only one diaphragm, leavingout. the rear diaphragm 30,Vbut Vthat would mean reduced volume of reproduction. yAlthough the frameFis so mounted that the diaphragm sheets are Supported along horizontallines and free along their vertical edges, it is obviousithat thesupporting frame Ffmay be held at any other angle.

Attention is called to the fact that the rectangular tensioned sheets 29and 30 constitute edge-free diaphragms adapted to vibrate withsubstantially' equal amplitude overtheir entire. acoustic area.Consequently,y these diaphragms respond with Vpractically uniformefiiciency at all frequencieswithin practical limits. The vib'ratoryareas of the ,dia-j phragms lie between the pairs of tensioning )ods 46and 47, respectively. Although the ensioned diaphragm `sheets 29 and 30ar-e ffery thin and therefore flexible, they vibrate i. :e stiff plateswithout nodal regions and are thus capable of giving a faithfulreproduction. [lf desired, one diaphragm may be of difierentmaterial ordimensions and un- 'ler a diiierent degree of tension from the therdiaphragm, so yas to respond most et ciently over a diderent band offrequencies.

made thinner and held under greater tension gthan the rear diaphragm 3G,thefront diahe lower tones. rlhe particular size and dinensions of thediaphragms are a. matter lthat can only be' determined by experimentwell within the skill of the trained artisan g toY give the best resultsin any particular dersien of instrument. f

Although l'have shown and described a specific constructiom l want itunderstood .l changes and modifications will occur to others withoutdepartingfrom' the scoper of theinvention as defined 1n the following 30the drawings do not showthe parts in their correct relative proportions,which have been purposely exaggerated for clearness, particularly thethickness'of the dielectric and the vibratory diaphragms.

I claim as my invention:

l. An a fixed condenser plate, a pair ot vibratory metal sheetsconstituting diaphragms arranged on opposite sides of said plate,elastic dielectric material separating said sheets fromV said plate,said sheets and plate being substantially fiat over their operatedareas, and means 'for supporting said sheets under tension andpressingthem against said dielectric material.

2. An electrostatic loudspeaker comprising a casing, a plurality` ofsubstantially flat condenserV plates in said casing, atleast one of saidplates consisting of a rect-angular vibratory sheet'V supported at twoopposite ends y only and held under tension, andinanually operable meansprojecting out of said casing for adjus ing the normal tension of'saidsheet.

a casing, a plurality of substantially fiat condenser plates in casing,Vatleast one of said plates consisting of arectangular vibratory sheetsupported at two opposite ends only and held under tension, and a rotarymember 60A'projecting out otsaid casing for adjusting the normal tension'of said sheet, said member being manually operable in either directionto increase and decreasesaid tension.

4. An electrostatic loudspeaker comprising l a fixed condenser plate, aypair of vibratory -nesoges or example, if the front diaphragm 29 isviphragm'will emphasize the higher tones,

vhile the other diaphragm will emphasize.

p that my invention is not limited to the de-y s tails set forth. It isto be expected that claims. lt is hardly necessary to add thatlelectrostatic loudspeaker comprising Y V3. An electrostatic loudspeakercomprising-` condenser :sheets 4'supported :at f opposite endsunder-tensiononeitherfsitle ofsaidlfixe'd plate and insuflated`therefrom,` fsaid tensionedA Y 5. Anelecta?ostati'cloudspeaker.compnising4 a iixed .condens'erplatd -a pairofvibratorymetal sheets situated on opposite sides of said plate inelectrostatic relation thereto, said plate and said sheets beingsubstantially rectangular, supporting means at opposite ends of eachsheet' to hold the same stretched, each i of said sheets consisting of adiaphragm, and means for adjusting at leastone ot the sup- Aportingmeans foreach diaphragm lto reg ulate the normal tensionthereof. Y

6. An electrostatic loudspeaker comprising 7. Ari-.electrostaticloudspeaker comprising` a 'fixed condenser plate, vibratory metal sheetarranged adjacent said fixed plate and constituting a diaphragm, saidplate and sheet being substantially rectangular, elastic dielectricmaterial between said plate and sheet, supports at opposite ends ofsaidsheet l to hold the same under tension, and a pair of cross-rodsengaging said sheet and so arranged as to press the same against thedielectric material.

8.r An electrostatic loudspeaker comprising a fixed .condenser plate, avibratory metal sheet arranged adjacent said fixed plate andconstituting a diaphragm, said plate andl sheet beingsubstantiallyrectangular, elastic sheet, supports at opposite ends ofsaid sheet te hold the same under tension, a pair ot crossrods engagingsaid sheet so that the sameis pressed against the dielectric material,and means for adjusting-the normal tension ofv said sheet.

y 9. An electrostatic loudspeaker comprising a fined condenser plate, avibratory metal sheet arranged adjacent said fixed plate and dielectricmaterial between said plate andl constituting ay diaphragm, said plateand i sheet being substantially rectangular, elastic dielectric materialbetween said plate and sheet, supports at opposite ends ofl said sheetto hold the same under tension, a pair of cross-rods engagingl saidsheet so that the same is pressed against the dielectric material, androtary means for adjusting at least one of said supports laterally ofsaid cross-bars

